The present invention relates generally to methods of liquid level measurement and related applications and more specifically to a liquid level measurement system and fuel transducer having an anisotropic magnetoresistance device that facilitates the measure of liquids, such as fuels in an aircraft fuel tanks.
Without limiting the scope of the invention, its background is described in connection with liquid level measurement in to fuel tank applications, more specifically aircraft fuel tanks.
Liquid level measurement in aircraft, automobiles, boats and other vehicles has historically been measured by either of two methods: float or capacitance probe. In each of these techniques, the fuel tank and its contents are subject to electrical energy in the measuring technique. Recently, there have been serious safety concerns due to unexplained aircraft losses which may have been caused by a spark from the electrical equipment inside the fuel tank. Other techniques such as air pressure, optics, and magnetic sensors have been proposed or implemented with varying success.
A method of liquid level measurement utilizing a Hall Effect semiconductor device is discussed in the Honeywell Solid State Sensors Catalog. Determining the height of a float is one method of measuring the level of liquid in a tank. For example, a linear output Hall Effect transducer can be placed outside the tank while a magnet is placed inside tank, and moved by the motion of a float arm. As the liquid level moves up or down, the magnet moves relative to the transducer, causing a change in transducer output voltage.
A Hall Effect transducer allows liquid level measurement without any electrical connections inside the tank. The use of a Hall Effect transducer, however, requires an electronic interface to allow the output to be used with a gauge. While there are linearity and temperature effects that must be either suppressed or compensated, a far more onerous problem is that the mechanical design must be such that a change in the strength of the sensed magnetic field due to mechanical slack does not indicate a change in quantity. Additionally, xe2x80x9crare earthxe2x80x9d magnets are generally required in any practical application.
Another method involves the use of a magnetic drive. In this method, two magnetic rotors are placed on either side of a non-ferrous plate which is used to cover an opening in either the side wall or top or bottom of the tank. The rotors, while on opposite sides of the plate, are arranged on axles each located on a common axis. Thus, when one rotor is turned, the other rotor follows the rotation due to the magnetic coupling between the two rotors. The torque available is in part a function of the offset of the poles (rotor diameter), magnetic strength (Gauss Level), and the distance between the rotors (Gap distance). Sufficient torque must be available to ensure close tracking of the following rotor and its readout device with the position of the driving rotor. This method does require multiple high strength xe2x80x9crare earthxe2x80x9d magnets which could make it impractical for high yield low-cost applications.
Accordingly, there is a need for a non-intrusive means of liquid level measurements that is cost-effective and does not require the use of expensive xe2x80x9crare earthxe2x80x9d magnets. Such a device would permit measurement of fuel levels in aircraft, boats, and other vehicles and would provide numerous advantages.
Disclosed in one enbodiment is a non-intrusive liquid level measurement system for measuring a quantity of liquid in a tank. The liquid level measurement system comprises an Anisotropic Magnetroresistance (AMR) device located outside the tank. The magnetic device is located inside the tank opposite of the AMR device with a mechanical interface providing a bridge between the AMR device and the magnetic device. The clearance between the AMR device and the Magnetic device is maintained such as that a magnetic field produced by the magnetic device can affect the resistance of the AMR device, the resistance being proportional to a measure of liquid in the tank. The measurement system can further comprise a float assembly coupled to the magnetic device in the form of a float arm that rotates about an arc in relation to the liquid in the tank, with the angle of the arc communicated to the magnetic device by rotational motion.
The system can further comprise an electrical interface communicably coupled to the AMR device, delivering a single output that is representative of a level of liquid in the tank. A physical connector can be used for delivering the voltage output to an external readout such as fuel gauge commonly found on an aircraft.
Further disclosed is a fuel quantity transducer for measuring the quantity of a fuel in a fuel tank. The transducer comprises an AMR device located outside the fuel tank and a magnet located inside the fuel tank. A flange provides a support between the AMR devivce and the magnet with the flange adapted to be attached to the wall of the fuel tank. The clearance between the AMR device and the magnet is maintained such that a magnetic field produced by the magnet can affect the resistance of the AMR device, the resistance being proportional to a measure of fuel in the fuel tank. The fuel quantity transducer can further comprise a support for holding the magnet in place within the fuel tank. The fuel quantity transducer can further comprise a row or coupled to the support and having two ends, a completed rod at one end and the magnet secured at the other end, the rotor adapted to rotational motion by an axis. A float arm is coupled to the rotor pivot arm and adapted for rotation about an arc in relation to the amount of fuel in the fuel tank. The rotor can take the form of a square-bar tapped axially at one end and counterbored for the magnet at the other end.
In one embodiment, the fuel quantity transducer includes a circuit board that comprises an interface for communication of an output signal related to the measured quantity of fuel tank and a plurality of electronic components that produce the output signal in relation to the properties of the AMR device. A cover can be placed over the electronic components to protect them from the elements and the environment.
An advantage of the invention is that it provides a non-intrusive means of liquid level measurement.
Another advantage of the invention is that the use of an AMR device eliminates the requirement of using expensive rare earth magnets.